Prior art photonic switching systems have used a plurality of planar devices to provide optical splitting and combining capabilities. One such arrangement, disclosed in U.S. Pat. No. 5,009,477, forms a crossbar switch having a stack of M splitter plates, each of which routes an injected optical signal to one of N outputs, and a stack of N combiner plates, each of which routes the signal of a selected one of the M inputs to a common output. Each splitter and combiner plate is formed from electro-optic elements, such as lithium niobate substrates. Another such switching arrangement, disclosed in a NASA Tech. Brief, dated September, 1990, employs an electronic shutter element wherein each electronic shutter in an array of such shutters can either be transparent or opaque to an optical signal. The electronic shutter element is planar and is disposed between optical signal splitting and signal combining devices. The signal splitting and combining devices are formed by etching quartz crystalline wafers.
The problem with the above-described structures is that they are quite expensive and utilize brittle, rigid materials which cannot meet the shock and vibration objectives of a number of optical switching applications.
U.S. Pat. No. 5,185,824 overcomes the above-mentioned problem by replacing the costly electrical-optic or fused quartz crystalline structures with molded plastic optical waveguide splitters and combiners. The optical switch is formed by arranging a stack of optical splitters and disposing this stack on one side of an electronic shutter element having an array of shutters. A similar stack of splitter elements is arranged, rotated 90 degrees with respect to the optical splitter stack and disposed on the other side of the electronic shutter element. The resulting switching structure provides an inexpensive, rugged device. However, this method is limited to use with highly collimated light beams or very large (approximately one millimeter in diameter), low bandwidth fibers. The reason is that, when used to switch light between pairs of fibers butted against optical flats on each side of the electronic shutter element, unless the fiber diameter is substantial compared with the thickness of the switching device which includes the optical flats, light collection efficiency is very low unless very expensive optics are used. The reason being that the diameters of standard silica fibers is very small compared with the thickness of optical flats required for high contrast switching elements. This problem can be resolved by utilizing expensive optics in the form of lenses on each end of the optical fibers. A second solution is the use of very large plastic optical fibers. Even so, optical losses are high and the low band width of these fibers will require optical-electrical-optical conversion at both the input and output stages. A third solution is to use very thin glass plates to get the fiber diameter/switch thickness ratio up. This suffers from the disadvantage that the contrast ratio and switching speed of the devices are impaired by the use of thin plates which are necessarily not as flat as the thicker plates. To make the optical plates thin enough to be used with standard multi-mode or single mode silica fibers would render the devices useless.
There exists then a need in the art for an optical switch that utilizes spatial light modulators which is inexpensive and physically strong and which can be utilized with the smaller diameter silica optical fibers.